In terms of quantities, the volume of annual production of fluoropolymers is not significant and is approximately one hundred thousand tones, which constitutes less than 0.1% of the world production of all polymers. However, in terms of cost, this segment of the market is significant, as it constitutes more than $2.5 billion and is steadily growing. In the production of fluoropolymer, the main share falls on PTFE (60 to 80%). The cost of fluoropolymers varies greatly. If for PTFE powders the cost per kilo ranges from tens of dollars, the cost per kilo for Nafion®-type films is as high as $50,000.
One tendency in the fluoropolymer market is the steady growth of high-tech products. Another tendency is the emergence of new fluoropolymer manufacturers, in particular, in China. These new producers are leading others in production volumes and are rapidly gaining popularity because of the low cost of their products.
On the other hand, a considerable volume of PTFE waste that sometimes reaches 40% of the production volume accumulates in the surrounding environment. Some specific features of polymer waste is stability against aggressive media, long periods of decomposition under natural conditions, and lack of decay. Most problematic in this respect are fluoroplastics, in particular, PTFEs, which, in fact, are valuable materials.
Production of PTFE micropowders in the USA, Russia, Western Europe, Japan, and China is measured in thousands of tons. For example, DuPont de Neumours & Company (USA) produces ultrafine powders of PTFE under the brand name Zonyl Fluoroadditive 1100 and Teflon® MP; DuPont Krytox (USA) produces granulated powders Teflon® PTFE; Lubrizol Corporation (USA) produces Fluotron dispersions; Western Reserve Chemical Corporation (USA) produces Powder PTFE Plastolon P-550; Shamrock Technologies, Inc. (USA) produces powders of Fluoro™ PTFE micronized series (average particle size of 1 to 2 μm); Russian companies Forum and Tomflon produce PTFE colloidal dispersions of the NanoFlon series and PTFE micropowders; and Dongyue Polymer Material Co., Ltd. (China) produces ultrafine PTFT powders having particle sizes that vary from 10 to 400 nm.
Main methods for industrial production of PTFE powders are based on polymerization of gaseous PTFE in aqueous media at predetermined technical conditions and with addition of appropriate reaction initiators. Particles obtained by such methods have dimensions ranging from 50 to 500 μm. By treating the powders in jet mills, particle diameter can be reduced from 50 to 10 μm.
Some methods of manufacturing PTFE powders involve radiation treatment of PTFE waste. Radiation causes accumulation of defects that initiate development of microcracks in the polymer. When the irradiated material is treated in jet mills, the particles break along these microcracks. The resulting particles have a molecular structure that completely corresponds to the structure of industrial samples of PTFE. For example, U.S. Pat. No. 7,482,393 issued in 2009 to C. Cody, et al, discloses a method for producing a submicron polytetrafluoroethylene (PTFE) powder in a free-flowing, readily dispersible form. The irradiated PTFE starting material is placed in a desired solvent and undergoes grinding until the PTFE particles reach submicron size. The submicron particles are subsequently recovered from the solvent and dried to form a powder that may have particles less than 1.00 μm in size. The dry PTFE powder may then be readily dispersed to submicron size into the desired application system. The submicron PTFE powder of this method is free flowing, readily dispersible in various application systems, and tends not to “dust” or self-agglomerate. Improved aqueous and organic dispersions of submicron PTFE particles may display increased stability and may require much less agitation than dispersions obtained by other processes. Such improved PTFE dispersions may be formed with or without the addition of surfactants, wetting agents, rheology modifiers, pH-adjusting agents, and the like.
German Patent No. 2315942 published in 1973 (inventor, Reinhard Neumann) discloses a method of manufacturing a granulated PTFE powder by mixing the PTFE powder disintegrated to particle size in the range of 0.1 to 0.5 mm with a liquid that is inert and nonwetting with respect to the PTFE and then drying the obtained granules at a temperature below the boiling point of the liquid. The initial material of the process comprises a polymer mixture obtained by suspension or emulsion polymerization. The particles are subjected to forces that occur during mixing.
Russian Patent RU 2133196 issued in 1999 (inventors, A. Uminskij, et al) discloses a method and apparatus according to which the apparatus is preliminarily flushed through with dry nitrogen and then filled with equal portions of a fluoroplastic waste. Batch melt is heated in a reactor, and hot fluoroplastic destruction products are cooled by a gas carrier and displaced into a tube. Products are deposited on walls and in a cooler and are then collected in receivers in the form of a powder, which is then packed. Remaining destruction material having been separated from the powder product is loaded into an afterburner, from which the resulting product is discharged through a discharge port for further processing.
An installation for recovering PTFE is described in Russian Patent RU2035308 (published in 1995; inventor, A. K. Tsvetnikov). The installation contains a reactor, a furnace, a screw-type feeder, cooling systems, and a fan. The reactor is provided with a cylindrical insert having a perforated bottom. The walls of the cylindrical insert are spaced from the inner walls of the reactor. The upper edge of the insert is arranged at the same level or below the level of the outlet openings of the reactor for discharge of the destruction products. Heating of the melt in the reactor to a temperature of 490 to 510° C. leads to thermal destruction, while the provision of a gap between the insert and the reactor walls makes it possible for the fan to blow the thermal-destruction products through a liquid-reaction phase. This increases gas-flow volume. The yield of the finely dispersed PTFE reaches 75%.
Russian Patent Application Publication 2000117474/03 published in 2002 (inventors, V. V. Panamarchuk and V. P. Deliya) discloses a method and a device for finely disintegrating powder materials. The method comprises acceleration of treated particles and simultaneously subjecting the particles to the effect of fields of centrifugal and pulsating-aerodynamic forces, wherein the aforementioned fields are generated without the use of external initiators. The aerodynamic field is created under the action of pumping blades and grinding rods at rotor rotation frequency in the range of 1500 to 3000 rpm. The field of aerodynamic forces has two turbulent counter-flows, and the zone of collision of two flows is additionally intersected by the openings of the centrifugal disk.
The apparatus for carrying out the above-described method comprises a disintegrating chamber formed by a vertically arranged housing and a vertically arranged rotor rotationally installed in the housing. The rotor is made in the form of a beam that is provided with vertically oriented stirring rods. The rotor comprises a drive shaft that rigidly supports the pumping blades and a centrifugal disk, which is provided with disintegrating rods. The pumping blades are located above the centrifugal disk and are inclined at an angle adjustable from 0 to 20°. A disadvantage of this device is insufficient degree of dispersion in the obtained product.
An apparatus for reprocessing PTFE is described in International Patent Application Publication WO 9847621 (A2) (1998, inventor A. F. Eryomin). The apparatus comprises a vortex rotary device that includes a housing containing a coaxially arranged rotor. The rotor has slots on its side surface and inlet and outlet pipe units for input and output of the treated material and a processing gas. The inner surface of the housing and the outer surface of the rotor are conical and equidistant. The side surface of the rotor has at least one annular rib that forms disintegrating chambers that communicate with each other.
In operation, the powder to be treated and the carrier gas are fed into the housing of the device through the respective pipe unions. In the course of rotation of the rotor, the slots of the rotor cause generation of intensive vortexes of the gas in the gap between the rotor and the inner walls of the housing. When the powder passes through the sequentially arranged disintegrating chambers, the powder particles collide with each other and disintegrate. The tapered shape of the rotor and the chamber provides intensification of the process in the direction from inlet to outlet.